Koutsoukis said: "He was at first angry, then exasperated that I wouldn't accept his denials at what I was putting to him.

''He told me he was like any other Australian who had made aliyah [immigration to Israel] and was trying to make a life in Israel.''

Fairfax Media spoke to Mr Zygier in Israel in early 2010 after learning that ASIO was investigating at least three dual Australian-Israeli citizens who had emigrated to Israel in the past decade. At the time, ASIO would not comment on the case. On Wednesday, the agency again refused to comment.

Each of the men had travelled back to Australia separately to change their names and obtain a new passport, two intelligence sources said at the time in Koutsoukis' story published in The Age.

One man had changed his name three times, and others had changed theirs twice, the source said, from names that identified them as European-Jewish to ones that were Anglo-Australian.

In each case, the men had used the new passports to travel to Iran, Syria and Lebanon - all countries that do not recognise Israel and do not allow Israelis, or anyone with an Israeli stamp in their passport, to enter. Israel also bans its citizens from travelling to these countries for security reasons.

Along with his Ben Zygier identity, he also used Ben Alon, Ben Allen and Benjamin Burrows.

At the time, Fairfax Media was investigating the men's involvement with a European communications company that had a subsidiary in the Middle East. The company's chief executive denied the men were ever employed by the organisation.

It is believed - although Fairfax Media has been unable to confirm - that Mr Zygier travelled back to Australia in 2009 to do an MBA at Monash University.

A source at the time observed him over several days sitting with a group of students from Saudi Arabia and Iran at the university's Caulfield campus.

The source said: ''[Australian Taxation Office] records from 2008 show that he applied for and was approved a HECS loan for postgraduate studies at Monash University where he is currently [November 2009] studying.''

Since 2006, Monash has been involved in education in Middle Eastern countries, and in 2007 it proposed an initiative for higher-degree students from Saudi Arabia.

Apart from his move to Israel and his MBA study, little is known about Mr Zygier's movements over the decade before he died, except that he was working in insurance law at the Australian firm Deacons in 2002.

In Israel Mr Zygier married a local woman with whom he had two children, the ABC reported.

It was well known that Israel approached people who emigrated from other countries to assist it by handing over their passports, an Israeli intelligence expert told Fairfax in 2010.

''Their names are used later but the person providing the passport is not involved,'' the expert said.

It is understood the ASIO investigation into Mr Zygier and the two other men began at least six months before the January 10, 2010, assassination of senior Hamas commander Mahmoud al-Mabhouh, widely believed to have been carried out by Mossad using Australian and European passports.

Three of those suspected of taking part in the assassination were travelling on Australian passports, using the names of dual Australian-Israeli citizens, authorities in Dubai confirmed.

There is no suggestion that the three Australian names linked to Mabhouh's assassination are connected to Mr Zygier or the other men investigated by ASIO.

After initially denying the Australian government had any knowledge that one of its citizens was detained in Israel, Foreign Minister Bob Carr said some officers in the Department of Foreign Affairs and Trade were aware of his detention.

The revelations raises questions about how much the Australian government knew about the conditions under which Mr Zygier was being held in the maximum security Ayalon Prison.

The ABC's Foreign Correspondent program, which named Mr Zygier as ''Prisoner X'', said he hanged himself in the specially constructed cell that was meant to be suicide proof.

Mr Zygier was held in isolation - and in secret - in Unit 15, a separate wing of Ayalon Prison that contains just a single cell in Israel's most secure prison in Ramla, near Tel Aviv.

The cell is believed to have been built for Yigal Amir, who murdered Prime Minister Yitzhak Rabin in November 1995.

It is described as a cell within a block within a prison. The inmate in Unit 15 was allowed no visitors and even after his death his identity was a state secret, protected by a court-issued gag order despite continuing protests from human rights groups.

''He is simply a person without a name and without an identity who has been placed in total and utter isolation from the outside world,'' a prison official was quoted as saying in the Israeli media in 2010, when news of ''Prisoner X'' first broke, and was then suppressed.

It is unclear what, if anything, Australia was told by the Israeli authorities about the death in custody of one of its citizens, or whether any consular assistance was provided to Mr Zygier during his time in solitary confinement.